The first person writes an answer, and the next person writes a question to that answer, and writes another answer for the next poster to write a question.
It sounds confusing but it's pretty simple.
Example:
Poster 1: A: You wish!
Poster 2: Q: Mum can i have my own country for my birthday?
A: WOW!! Thats so cool!
(Also, be sure to mark them something like i did in example. A is answer, Q is question)
Ok i start....
A: That's impossible...